If it is your intention to live in Portugal for longer than 3 months and you are a citizen of the EU, Switzerland, Iceland, Liechtenstein, Norway and Andorra, you will need a certificate of registration to formalize your stay in Portugal. For as long as the UK is part of the EU, this applies to UK citizens as well.
The website http://www.sef.pt/portal/v10/PT/aspx/apoioCliente/detalheApoio.aspx?fromIndex=0&id_Linha=4351 with an English option, is very useful.
You must apply for this certificate within 30 days after the expiry of the original 3 months’ period of residence in Portugal.
The town council offices where you reside will provide you with such a certificate upon payment of a fee (as at July 2016, €15-00). In Cascais, the Loja de Cascais in Rua Manuel Joaquim Avelar, nº 118, in the same building as the main Post Office (level -1), will issue the certificate of residence.
The following documents must be produced:
Family members will also need certificates of residence. If they are EU citizens or citizens of Switzerland, Iceland, Liechtenstein, Norway and Andorra, they will need to produce the following:
If those accompanying you are over 21 years such as direct descendants over 21 years or direct ascendants(parents/grandparents, proof that you are still responsible for them is required. If they are not directly related to you, you need to provide a certificate from your own country (or country in which you previously lived), stating that you are responsible for the person concerned and you need to justify why the person must live under the same roof as you.
This will be your first step to becoming a full resident in Portugal. After 5 years you will have to apply for a permanent residence.